Fluid-cooling slow wave interaction structure for a traveling wave tube

ABSTRACT

A helical slow wave structure formed of hollow metal tubing communicating with inlet and outlet fluid manifolds through supporting hollow ceramic tubes brazed to the helix at every half turn. Fluid coolant flows from the inlet manifold through the ceramic tubes and into each half turn of the helix and then out through the ceramic tubes on the other side of the outlet manifold where it is pumped to a heat exchanger and then returned to the inlet manifold.

United States Patent References Cited UNITED STATES PATENTS Theodore J. Marchese North Springfield;

[ 72] Inventors Sidney T. Smith, Alexandria, Va.; Henry D.

Attorneys- Arthur L. Branning, James G. Murray and John M. Neary, R. S. Sciascia [54] ABSTRACT: A helical slow wave structure formed of hollow 6 Cmms 3 Dn'hg g metal tubing communicating with inlet and outlet fluid manifolds through supporting hollow ceramic tubes brazed to the helix at every half turn. Fluid coolant flows from the inlet manifold through the ceramic tubes and into each half turn of the helix and then out through the ceramic tubes on the other side of the outlet manifold where it is pumped to a heat exchanger and then returned to the inlet manifold.

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FLUID-COOLING SLOW WAVE INTERACTION STRUCTURE FOR-A TRAVELING WAVE TUBE STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates to traveling wave tubes and particularly to an arrangement for cooling a high-power, wide bandwidth, slow wave interaction structure.

In power applications and particularly in electronic warfare applications, octave bandwidth traveling wave tubes capable of amplifying radiofrequency energy at the kilowatt level are urgently needed. The structure presently performing the needed amplification is well-known. It comprises a helical conductor for radiofrequency electromagnetic waves disposed in a vacuum tube having at one end an electron gun for projecting an electron beam along the axis of the helix. The interaction between the electron beam and the helically propagated RF wave causes amplification of the wave.

At present, the primary factor limiting increased tube power output is the inability to dissipate heat generated in the slow wave structure. This problem is well-known in the prior art and attempts at its solution are legion. One of the most common techniques for dissipation of the heat is to support the helix within the vacuum envelope by means of three or more dielectric rods spaced equally around the helix parallel to its axis and in contact with the vacuum envelope and the helix. This arrangement provides both mechanical support for and alignment of the helix within the envelope and a thermal path therebetween. The wide use of this technique attests to its general acceptance by the industry. However, this acceptance is qualified by some very severe performance limitations. One limitation is the low heat dissipation capability of this arrangement. The area of contact between the support rods and the helix is very small thus limiting the heat conduction between the helix and the rod to a low rate. Moreover, the rods are necessarily formed of ceramic material which has a low thermal conductivity thereby further limiting the conduction of heat to the vacuum envelope. Attempts at increasing the area of contact and substituting material having a higher thennal conductivity have been of limited success because of the adverse effects on the electrical characteristics of the tube.

A second problem arising from the rod support technique is in the increased dielectric loading and lowered interaction impedance. Microwave dielectric loading due to the presence of the dielectric rods disposed alongside the helix decreases the efficiency and gain of the tube and decreases its operating bandwidth. Attempts at reducing the dielectric loading by recessing' the supporting structure and decreasing its area of contact with the helix necessarily result in a decrease in the already low thermal conductivity of the thermal path between the helix and the vacuum envelope. It has therefore been necessary in the past to devise a compromise configuration which would provide acceptable heat conduction through the dielectric supports while holding the level of dielectric loading as low as possible. The compromise obviously does not provide a satisfactory solution to either problem.

Another technique for cooling the helix attempted in the prior art is to form the helix of metal tubing and pump a fluid coolant throughout the length of the helix. The elegant simplicity of this technique is immediately appealing, but in practice it has not worked well for high frequency traveling wave tubes because the metal tubing must be very fine and therefore the volume rate of fluid coolant it can handle is necessarily very low. Consequently the quantity of heat that may be removed by the coolant is also very low.

One final technique attempted in the prior art was to make the ceramic rod supports themselves hollow and conduct a fluid coolant through the rod supports. This technique increased the heat conduction capability of the rod supports, but the area of contact between the helix and the rods remained small and therefore the heat conduction between the helix and the rods was not increased sufficiently to solve the problem.

The art therefore has long been in need of a slow wave interaction structure which provides a wide bandwidth operating range at high-power with low dielectric loading and high interaction impedance. The helix must be rigidly supported with each helix turn held immobile with respect to the axis and each other turn, and provision must be made for high-capacity heat dissipation from the helix.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide a fluid-cooled, slow wave interaction structure for a traveling wave tube having a wide-bandwidth operating range at highpower output levels.

Another object of the present inventionis to provide a fluidcooled, slow wave interaction structure for a traveling wave tube having strong mechanical support for the helix wherein each helix turn is held rigid with respect to each other and the helix axis.

A further object of the present invention is to provide a fluid-cooled, slow wave interaction structure for a traveling wave tube having high operating efficiency and gain.

Still another object of the present invention is to provide a fluid-cooled, slow wave interaction structure for a traveling wave tube having a high heat-dissipation capability for cooling the slow wave interaction structure.

A still further object of the present invention is to provide a fluid-cooled slow wave interaction structure for a traveling wave tube capable of operating over wide bandwidths at high frequencies and at high-power levels.

Briefly, these and other objects are attained by providing a slow wave interaction structure formed of hollow metal tubing having holes formed in the outside of the helix tubing at each half turn. A short hollow ceramic tube is brazed into each helix hole, and the outside end of each ceramic tube on one side of the helix communicates with an inlet fluid manifold and the outside end of each ceramic tube on the other side of the helix communicates with an outlet fluid manifold. In use, the fluid is pumped into the inlet fluid manifold and passes through the series of short ceramic tubes communicating therewith to each turn on one side of the helix. The fluid flows through each half turn of the helix and then immediately is conveyed out therefrom via the short ceramic tubes communicating between the other side thereof and the outlet fluid manifold. Since the fluid flows through only a very short length of the helix, i.e., one-half turn, and since there are twice as many such fluid paths as there are turns of the helix, the volume rate of fluid flow which the helix can handle is increased manyfold and the heat conduction capability of the fluid-cooling system is commensurately increased.

DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and its many attendant advantages will develop as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a sectional elevation of the slow wave interaction structure in a'traveling wave tube according to the present invention;

FIG. 2 is a sectional elevation along lines 2-2 in FIG. 1; and

H6. 3 is a sectional end view of a second embodiment of a slow wave interaction structure according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference characters designate identical or corresponding parts throughout the several views and more particularly to FIG. 1, a section of a traveling wave tube is shown having a vacuum envelope 12 formed of a nonmagnetic material such as Monel or copper. Coaxially disposed within envelope 12 is a copper sleeve 14 formed of two semicylindrical portions best seen in FIG. 2, joined at diametrically opposed longitudinal seams 16 which are both brazed to form a high-pressure fluid seal 18. Metal sleeve 14 is supported in its axial position within envelope 12 by a pair of longitudinal dividers or blocks 20 shaped on the outside to match the inside contour of vacuum envelope l2 and shaped on the inside thereof to match the outside surface of sleeve 14. In blocks 20 are formed longitudinal recesses 22 which receive brazing material to form a high-pressure fluid seal which becomes integral with seal 18 when the composite structure is heated in a furnace. On the outside surface of blocks 20, a groove 24 is milled which receives brazing material to form a highpressure fluid seal with the wall of envelope 12 when heated in the furnace. Blocks 20, vacuum envelope 12, and metal sleeve 14 define a pair of semiannular cavities 26 and 28 which are fluid tight with respect to one another and function as intake and exhaust fluid manifolds in a manner to be described later.

A helix 30 of metal tubing, such as annealed copper, is axially disposed within metal sleeve 14. A series of holds aligned in a pair of diametrically opposed longitudinal rows along the outside of the helix is formed communicating with the interior of the tubing. Thus each turn of the helix has a pair of apertures formed in the outside wall thereof and the apertures are disposed in a pair of diametrically opposed rows running the length of the helix. The technique for forming these apertures, which must be very precisely formed, is the spark erosion technique. This technique permits the drilling extremely precisely defined apertures in very fine metal tubing without generation of excessive heat which would destroy the tubing.

A series of hollow, short, ceramic tubes 32 and 34 having tapered inner ends are brazed in the series of holes formed in the outside wall of the helix 30. The tapering of the ends of ceramic tubes 32 and 34 facilitates centering of the tubes in the apertures formed in helix 30. Brazing of each tube around the entire junction thereof with the helix forms a high-pressure fluidtight seal therebetween, thereby establishing fluid communication between tubes 32 and 34 and helix 30. The tapering of the ends of tubes 32 and 34 also provides a stronger mechanical connection between the tubes and the helix, for, during the brazing operation, the tubes are urged into the apertures by springs on the brazing jig and as the helix gets hot, the apertures widen thereby allowing the tubes to slip somewhat deeper into the apertures. Then when the brazing is complete and the helix is cooled, the copper contracts more than the ceramic pipes and shrinks tightly around the end of the pipe, thus enhancing the mechanical joint and militating for fluid integrity.

The two longitudinal halves of metal sleeve 14 are then fitted together around helix 30 with tubes 32 and 34 extending through a series of apertures previously formed in the metal sleeve halves. With the sleeve halves and blocks clamped together in a brazing jig, the assembly is passed through the furnace and pipes 32 and 34 are brazed to copper sleeve 14 around the junction therebetween, and the braze at seam l6 and recess 22 is accomplished. The brazed assembly is inserted into vacuum envelope 12 with brazing material in groove 24 and the'entire tube is reheated in the furnace to form the last braze and complete the assembly.

It should be understood that most of the heat generated is near the output end of the helix and, in some designs, near the attenuator portion of the helix. It therefore may be prudent in a practical embodiment of this invention to provide fluid flow only in the high heat regions of the helix.

In order that the braze between the tubes 32 and 34 and the helix 30 not melt when subsequent brazes are made, the brazing material is of a higher melting temperature than that for the other brazes. A gold-copper eutectic having a melting point of l,040 C. has been found to work well. Similarly, the

brazing material for the seam l6 and recess 22 should have a higher melting temperature than that for groove 24 for the same reason.

In operation, a radiofrequency wave is introduced into helix 30 at an inlet microwave coupling (not shown). A beam of electrons from an electron gun (not shown) is directed axially through the middle of the helix and is collected at the exit end by a collector anode (also not shown). The radiofrequency wave traveling along helix 30 is amplified by interaction with the electron beam traveling down the axis of the helix 30 in a manner well-known in the art. The amplified wave is then collected at a conventional coupler (not shown).

The heat generated by radiofrequency losses in the helix and impingement of the electron beam on the helix is removed from the helix in the following manner: Liquid coolant, such as low RF loss cooling oil FC-43, 75, 77 made by 3 M Company or DC-200 made by Dow Corning Company is admitted to intake manifold 26 through a fluid line shown schematically at 42 running from a heat exchanger (not shown) and a pump (also not shown). The coolant fills and pressurizes intake mainfold 26 and is admitted to one side of each turn of helix 30 by means of ceramic tubes 32 which serve as a fluid conduits between manifold 26 and helix 30. The fluid then flows around each half turn of helix 30 and is exhausted to exhaust manifold 28 by means ceramic tubes 3%. The now warm fluid-cooling oil is then withdrawn from exhaust manifold 28 by outlet fluid line shown schematically at 44 and cycled back 'to the previously mentioned heat exchanger.

The increased heat dissipation capability of this invention is self-evident. The increased volume of cooling fluid which the helix can handle by having the fluid traverse only a single half turn of the helix provides a cooling capability approximately 30 times greater then conventional conducting ceramic rod supports. In addition to this dramatic increase in cooling capacity, an additional benefit contributed by this arrangement is a remarkably lower level of microwave dielectric loading of the helix. This is due to the fact that the supporting ceramic tubes 32 and 34 extend radially away from the helix instead of lying longitudinally alongside as previously done in the prior art. This lower dielectric loading increases the efficiency, broadens the bandwidth, and raises the gain of the traveling wave tube and, in connection with the greatly increased cooling capacity, results in a traveling wave tube of much greater flexibility and effectiveness.

Looking new at FIG. 3, another traveling wave tube constructed in accordance with this invention includes a pair of ceramic pipes 46 and 48 disposed longitudinally within vacuum envelope 12 on opposite sides of helix 30. The interiors of pipes 46 and 48 communicate with the interior of helix 30 via a conduit 49 formed by a series of apertures formed therethrough at each half turn of the helix and a matching series of apertures formed through pipes 46 and 48. A braze 50 surrounds the junction of pipes 46 and 48 with the helix thereby mechanically joining the helix to the two ceramic pipes while establishing a series of fluidtight junctions such that fluid pumped underpressure into ceramic pipe 46 flows into the helix at each half turn, flows around each half turn of the helix, and exits directly into ceramic pipe 48. The assembly is inserted within vacuum envelope 12 ad the device functions similarly to that shown in FIGS. 1 and 2. The dielectric loading is somewhat higher in the embodiment shown in FIG. 3 but the assembly procedures are simpler since there is only one braze per junction instead of two as is the case for the embodiment of FIGS. 1 and 2. Moreover, the completed assembly provides a somewhat stronger and mechanically more reliable structure.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.

What is claimed and desired to be secured by Letters Patent of the United States is:

ing said sleeve within said envelope;

wherein said inlet manifold and said outlet manifold are defined by respective sides of said divider means, the interior wall of said envelope and the exterior wall of said 1. A traveling wave tube having a vacuum envelope and disposed therein a helical slow wave structure formed of hollow tubing, wherein the improvement resides in structure for cooling said helix, comprising:

an inlet fluid manifold; 5 sleeve. an outlet fluid manifold; 5. The tube defined in claim 4, wherein: inlet means for establishing fluid communication between Said inlet means comprises a plurality of nonmetallic tubes, said inlet fluid manifold and said helix at every turn of each connected at one end thereof to one side of said portions of aid h li helix and communicating with the interior of said helix outlet means for establishing fluid communication between 10 l i and connecled at f f end thereof to the said helix and said fluid manifold at every turn of said porof Sam sleeve and communicating themthmughi and tions f said helix; said outlet means comprises a plurality of nonmetallic whereby cooling fluid may be pumped into said inlet fluid tubes; connected at flfereof P thmhersiqe manifold, thence to be divided by said inlet means to take of i and commmflcatmg the manor of saw a plurality of paths a portion of said fluid flowing through helix tub ng and connection at the other end there to and each half turn of said portion of said helix, thence to be cpmmimlcating through the wall. of Sald sleieve at a recombined by said outlet means in said outlet fluid i lamemcany opposed to sand connecuon thereto of manifold. said inlet tubes. 2 The tube defined in claim 1 wherein The tube defined m clam when:

said inlet and outlet manifolds comprise a pair of nonmetalsald mlet.means compnses a plurality onomi1etanl.c times he pipes dispose longitudinally within said envelope extending between and communicating with said inlet parallel to and abutting Said helix; .mamfold and Said hem; f said inlet means comprises means defining a first set of aper outlet compnses a plurahty nonm'agnetfc tures through the wall of said helix tubing communicating extenfhng and commumcatmg Sald with the interior thereof and means defining a set of aper- 3 3:35:: z rg g i g sx g t tures through the wall of said inlet manifold pipe, said a series of holes are formed through the wall of said helix 23:; 2:22:25? z g jzgg z g i il gg ggf 25:2 mbmg; which fluid may pass from said inlet manifold pipe to said the ends of said nonmagnetic tubes are tapered and fit into helix tubing;

Said h1e5 in said helix tubing; all said outlet means comprises means defining a second set of the junction between said nonmagnetic tubes and said helix apertures h h h ll of id h m bi on h id tubing is brazed to form 8 gh-pr fluidlight Seal of said helix diametrically opposed to said first set of and establish fluid communication between said tubes apertures and communicatin with the interior of said d id h li bi helix tubing, and means de ming a set of apertures 4. The tube defined in claim 1, further comprising: thmugh the wall of Said outlet manifOld P p said PF a nonmagnetic metal sleeve disposed within said envelope tures in said Second Set and said Juliet manifold P P and coaxially around Said helix; apertures being aligned and forming a second set of cana pair of divider means, a respective one of each disposed duits through which fluld y P m said helix to 531d between and sealed to respective opposite sides of said 4 outlet mamfold Plpesleeve and the interior wall of said envelope for support- 

1. A traveling wave tube having a vacuum envelope and disposed therein a helical slow wave structure formed of hollow tubing, wherein the improvement resides in structure for cooling said helix, comprising: an inlet fluid manifold; an outlet fluid manifold; inlet means for establishing fluid communication between said inlet fluid manifold and said helix at every turn of portions of said helix; outlet means for establishing fluid communication between said helix and said fluid manifold at every turn of said portions of said helix; whereby cooling fluid may be pumped into said inlet fluid manifold, thence to be divided by said inlet means to take a plurality of paths, a portion of said fluid flowing through each half turn of said portion of said helix, thence to be recombined by said outlet means in said outlet fluid manifold.
 2. The tube defined in claim 1, wherein: said inlet means comprises a plurality of nonmetallic tubes extending between and communicating with said inlet manifold and said helix; and said outlet means comprises a plurality of nonmagnetic tubes extending between and communicating with said helix and said outlet manifold.
 3. The tube defined in claim 2, wherein: a series of holes are formed through the wall of said helix tubing; the ends of said nonmagnetic tubes are tapered and fit into said holes in said helix tubing; an the junction between said nonmagnetic tubes and said helix tubing is brazed to form a high-pressure fluidtight seal and establish fluid communication between said tubes and said helix tubing.
 4. The tube defined in claim 1, further comprising: a nonmagnetic metal sleeve disposed within said envelope and coaxially around said helix; a pair of divider means, a respective one of each disposed between and sealed to respective opposite sides of said sleeve and the interior wall of said envelope for supporting said sleeve within said envelope; wherein said inlet manifold and said outlet manifold are defined by respective sides of said divider means, the interior wall of said envelope and the exterior wall of said sleeve.
 5. The tube defined in claim 4, wherein: said inlet means comprises a plurality of nonmetallic tubes, each connected at one end thereof to one side of said helix and communicating with the interior of said helix tubing, and connected at the other end thereof to the wall of said sleeve and communicating therethrough; and said outlet means comprises a plurality of nonmetallic tubes, each connected at one end thereof to the otherside of said helix and communicating with the interior of said helix tubing, and connection at the other end there to and communicating through the wall of said sleeve at a position diametrically opposed to said connection thereto of said inlet tubes.
 6. The tube defined in claim 1, wherein: said inlet and outlet manifolds comprise a pair of nonmetallic pipes dispose longitudinally within said envelope parallel to and abutting said helix; said inlet means comprises means defining a first set of apertures through the wall of said helix tubing communicating with the interior thereof and means defining a set of apertures through the wall of said inlet manifold pipe, said helix apertures and said inlet manifold pipe apertures being aligned and forming a first set of conduits through which fluid may pass from said inlet manifold pipe to said helix tubing; said outlet means comprises means defining a second set of apertures through the wall of said helix tubing on the side of said helix diametrically opposed to said first set of apertures and communicating with the interior of said helix tubing, and means defining a set of apertures through the wall of said outlet manifold pipe, said apertures in said second set and said outlet manifold pipe apertures being aligned and forming a second set of conduits through which fluid may pass from said helix to said outlet manifold pipe. 